This invention relates to bubble memory systems. More particularly, this invention relates to bubble memory systems suitable for mass storage.
Bubble memories enjoy the advantage of having no mechanical or moving parts and therefore are considered to be of high reliability. However, costs have been relatively high and speed of operation for serial reading of data from a loop in a bubble memory has generally been rather low. For example, most bubble memories are limited to data read-out rates of approximately 12.5K bytes/second. This does not compare favorably to disc drives, which can read out data at rates of up to 500K bytes/second (significantly more than an order of magnitude faster). Thus, the preferred device for inexpensive mass storage in data processing systems has generally been a hard or floppy disc device.
In most applications, data is not read out continuously. Instead, relatively small blocks of data are read from various "pages" of the memory. Then a different portion of memory is accessed, and another relatively small block of data is read. Thus, a major limitation in the speed of operation of a computer system is the amount of time necessary to access the data in mass storage. For example, for a disc drive the time required to move the read head, permit it to stabilize in a new position with respect to the disc, and begin reading data, may be in the order of 50 msec. On the other hand, in a bubble memory device using bubble chips having a major loop and minor loops with 2,000 bits each, at a rate of 100K bits/second, the worst case access time for reading out data would be approximately twenty milliseconds (10 microseconds/bit.times.2,000 bits) and the average access time would be approximately ten to twelve milliseconds. Thus, if the read out rate for bubble memories could be improved by a factor of just under an order of magnitude, bubble memories would favorably compete with disc drives in terms of overall speed of operation.
Bubble memory chips also have another disadvantage in that not all minor loops on a bubble memory chip are usable. When a chip is tested over the full range of operating temperatures, generally a small percentage of the loops fail to perform properly. The use of these loops would introduce unacceptable errors into the data.
Generally, at the factory, data concerning which loops are inoperative is written into certain of the loops (which must test good at the factory). This bad loop data (BLD) is stored in "boot loops" on the chip. It is also recorded on the outside of the chip so that the data can be written back on to the boot loops should it become necessary to erase and reload all of the information on the chip.
The BLD is used to instruct a memory system controller to avoid writing data into and reading data from the bad loops. Generally, the BLD for a single chip is accessed on the chip and is loaded into a memory associated with the controller for only that chip.
In mass storage systems, many chips may be used. A central controller must access and use the BLD of many chips. When the controller switches from one chip to another, the BLD for the new chip must be accessed, and then supplied to the controller. This additional access time interferes with the operation of the system and tends to make the system less desirable.